In an offset printer, a series of rollers transfers ink in the form of an image from roller to roller until the ink is finally transferred onto a media, such as paper. The media is fed into a printing nip or gap formed between two rollers, sometimes referred to as a transfer roller and a media roller. In some instances, the transfer roller comprises a blanket, such as an electrically conductive rubber-coated fabric, for transferring the ink to the media. This blanket is typically secured to a cylinder of the transfer roller via a clamp or other fastening mechanism, which introduces a seam or discontinuity on the surface of the transfer roller. The media roller oftentimes comprises another clamp or fastening mechanism for fastening or attaching the media, resulting in a further seam or discontinuity on the surface of the media roller.
The seams may disrupt the uniform pressure between the transfer roller and the media roller. This can be addressed by varying the relative position of the first roller and the second roller when a seam passes through the interaction zone in which the rollers are in rolling contact. By choosing a suitable gap profile, the size of the gap between the first roller and the second roller may be adjusted to compensate for the seams on the first roller and/or the second roller, respectively, thereby reducing disruptions caused by the seams.
While these techniques help to enhance the printing quality, problems remain when media of varying lengths are printed consecutively. In particular, after printing media of a given size, frame marks or paper size marks develop on the surface of the blanket. These frame marks are coincident with the media edges and may be attributed to mechanical abrasion or chemical changes of the blanket layers. These engravings may result in undesired paper size marks on subsequently printed longer printer media. To enable printing on the subsequent larger paper size, the blanket needs to be replaced, resulting in higher printing costs, reduced blanket life spans, increased press down time, and loss of productivity as well as increased paper waste.
It is possible to use different blankets for different media lengths. However, this requires the user to alternate between several blankets, which is time-consuming and awkward.
The effect of the paper size marks can also be alleviated by printing long media before short media. However, this requires a lot of advance planning; thereby reducing the degrees of freedom generally associates with digital printing.
The gap profile of the prior art may usually require a certain rise time and therefore may overlap with the media to be printed. These overlaps may lead to degradations of the printing quality when images are printed close to the media edge at which the overlap occurs.